1. Field of the Invention
The invention relates to a process for purifying exhaust gas from a combustion engine.
The invention is specifically directed to remove unburned hydrocarbons, soot and carbon monoxide from a sulphur dioxide containing exhaust gas collected from one or more diesel engines.
The invention is especially useful in diesel engines, where the fuel has a high content of sulphur compounds.
2. Description of Related Art
Purification of exhaust gasses is already known in the art.
Removal of NOx and CO from an exhaust gas is described in EP 0 208 434, where NH3 is added, and NOx and CO are removed in contact with a catalyst.
In EP 0 411 094 a process is disclosed, where NOx is converted by means of NH3 to N2 in a catalyst with two layers, where also CO is converted. However, presence of SO2 creates the risk of ammonia sulphate formation.
Purification of exhaust gas is also disclosed in U.S. Pat. No. 5,591,414, where the purification is performed in presence of a catalyst, which also acts as a sorption filter for removing remaining hydrocarbons simultaneously with oxidation of nitrogen oxides and carbon monoxide. The catalyst is of a honeycomb structure and has two catalytic layers.
Unburned hydrocarbons, carbon monoxide and nitrogen oxides are also converted in a process described in U.S. Pat. No. 6,214,307. This takes place in the presence of a catalyst comprising two layers of different composition active in reduction of NOx and oxidation of hydrocarbon and CO.
Prior art, however, does not mention the problem of removing HC, CO and NOx without SO2 interfering by being oxidised to SO3. SO3 creates the risk of formation of solid sulphates or a mist of sulphuric acid. This oxidation takes place simultaneously with the other oxidation processes.
It is the object of this invention to provide a process, where hydrocarbon and carbon monoxide impurities in a sulphur containing exhaust gas are converted by oxidation without creating SO3 and thereby a mist of sulphuric acid.